This invention relates generally to rectifier circuits used as a power source suitable for discharge lamps or the like, and particularly, the present invention relates to rectifier circuits of high efficiency and high power factor.
As a lighting or driving device for use with a discharge lamp, such as a fluorescent lamp, is known a high-frequency lighting device which is advantageous for improving luminous efficiency, for reducing loss in electrical power, and for reducing the size of its stabilizer. In such a high-frequency lighting device, as shown in FIG. 1, a power source having a well-known rectifier circuit, which comprises a rectifier and a capacitor, is used such that an input AC voltage V.sub.AC (see FIG. 2 waveform A) applied across input terminals 3 and 4 is rectified by way of a rectifier 1, and then the rectified voltage is smoothed by way of a smoothing capacitor 2 so as to develop a DC voltage V.sub.DC across output terminals 5 and 6 (see FIG. 2 waveform B). This output DC voltage V.sub.DC is used to drive a high-frequency oscillator (not shown) which energizes a discharge lamp (not shown), and it is generally known in the art that such a high-frequency lighting device provides 15 to 20 percent higher luminous efficiency when compared to low-frequency lighting technique using 50 or 60 Hz commercial AC source.
However, in the above-mentioned rectifier circuit of FIG. 1, since the smoothing capacitor 2 is charged with a charging current only when the input AC voltage V.sub.AC exceeds the voltage across the smoothing capacitor 2, an input alternating current I.sub.AC supplied to the rectifier 1 has a pulse-like waveform. As a result, the power factor viewed from the AC source shows a very low value, such as 50-60 percent, which is undesirable in view of working efficiency of wirings.
In order to obviate such a drawback, therefore, the smoothing capacitor 2 of FIG. 1 may be removed from the rectifying circuit, and such a rectifying circuit without a smoothing capacitor is disclosed in TOSHIBA REVIEW VOL. 34, No. 7, page 585 (published on July 1, 1979), titled "New Series of Electronic energy-saving ballasts, super ballasts". FIG. 3 shows such a conventional arrangement, while FIG. 4 is a waveform diagram showing the operation of the arrangement of FIG. 3. In the arrangement of FIG. 3, since no smoothing capacitor is employed, an output DC voltage V.sub.DC developed across the output terminals 5 and 6 includes ripples (see FIG. 4 waveform B) so that the high-frequency oscillator responsive to the output voltage V.sub.DC is driven by a ripple voltage. Due to the absence of a smoothing capacitor, the input alternating current I.sub.AC is a sinusoidal wave as shown in FIG. 4 waveform C, and therefore, power factor is improved when compared to the arrangement of FIG. 1.
However, the arrangement of FIG. 3 suffers from a low luminous efficiency and low effective current as follows. Namely, in the arrangement of FIG. 3 the lamp current flowing through the discharge lamp or tube is difficult to immediately rise if it is made zero and if sufficient energy is not supplied thereto thereafter. Such a slow or gentle rising of the lamp current results in the presence of a duration in which an effective current does not flow through the discharge tube, which duration will be referred to as a dark angle region hereinafter. During such a dark angle region or period, are discharge within the discharge tube is interrupted, resulting in decrease in luminous flux and therefore in brightness. Thus, the luminous efficiency expressed in terms of Lm/Win of the discharge lamp becomes low, wherein Lm is the luminous flux emitted from the discharge lamp, and Win is the consumed power viewed from the power source.
The present inventor has divised a rectifier circuit in which such a dark angle region is eliminated for improving luminous efficiency without deteriorating the power factor, prior to the present invention and filed a patent application No. 56-195978 at Japanese Patent Office. According to the inventor's prior rectifier circuit, a capacitor having an extremely small electrostatic capacitance, such as 15 .mu.F or less, is provided to the output side of the recitfier circuit, and the discharging period of the capacitor is set to the dark angle region which appears when a discharge tube is driven by only the rectifier so that the dark angle region is filled with a discharging current from the capacitor. With this arrangement, it is possible to improve the luminous efficiency without deteriorating the power factor.
However, when it is intended to further improve the power factor with the above arrangement, the electrostatic capacitance of the capacitor has to be reduced, while an extremely small capacitance results in the occurrence of small-current regions which lower luminous efficiency. Furthermore, since the capacitor is charged, the waveform of the input AC turns such that a pulse-like wave is superposed upon a gradually reducing wave, and therefore, the power factor is lowered thereby. Namely, there is a limit in power factor improvement according to the above structure.
In order to remove dark angle regions with a high power factor being maintained, the output voltage from a rectifier may be applied, as disclosed in U.S. Pat. No. 4,109,307, to a capacitor via an element having a large inductance, while the capacitor is discharged via a diode. In this arrangement, although power factor is improved since the input alternating current takes a waveform of a gentle rectangle inasmuch as pulses are merely included, there is a disadvantage that power consumption is large due to ohmic loss consumed by a winding conductor which constitutes the above-mentioned inductance element.